Truncation of the cytoplasmic domain of the simian immunodeficiency virus envelope glycoprotein increases env incorporation into particles and fusogenicity and infectivity.

Growth of macaque simian immunodeficiency virus (SIVmac) in certain cloned human T-cell lines, such as HUT.78, selects for isolates containing a premature stop codon within the cytoplasmic domain of the transmembrane envelope glycoprotein. In contrast, propagation of virus in macaques or in their cultured T cells favors replication of virus containing the full-length envelope glycoprotein. To elucidate the causes of this phenomenon, we used a human immunodeficiency virus pseudotyping system to assess the effects on infectivity of the cytoplasmic domains of envelope glycoproteins obtained from SIVmac1A11 and SIVmac239. These envelopes contain truncated and full-length cytoplasmic domains, respectively. By analyzing human immunodeficiency virus particles containing selectable genes pseudotyped with each glycoprotein or with chimeric derivatives, we found that truncation of the cytoplasmic domain resulted in a significant advantage in viral entry into HUT.78 T cells and CD4+ U87.MG glial cells. Truncation of the cytoplasmic domain significantly enhanced both envelope density on particles and envelope-mediated cell-to-cell fusion. It is likely that one or both of these effects contribute to the observed differences in infectivity and to the selection of virions with short cytoplasmic tails in human T cells.